The present invention relates generally to woodworking joints and drilling jigs used to produce them. More particularly, this invention relates to loose tenon joints and drilling jigs used to drill a mortise. In a loose tenon joint, both pieces of stock to be joined are mortised (slotted) and a section of precut tenon is inserted into the mortises securing the two members.
Articles appearing in "Fine Woodworking" magazine of March/April 1995, (pages 58-61) and January/February 1993 (pages 46-48) provide background of various mortise and tenon options, including loose tenon joinery. Some typical applications using loose tenon joints are doors, windows and cabinet casework. In general, a loose tenon joint is comprised of two wood members with similar sized mortises formed in each piece. An independent loose tenon member (matching the size of the mortises) is captured by the two mortises securing the joint. Conventional mortises have parallel planar surfaces with either square or rounded corners. Tenon stock can be purchased or prepared by the woodworker. Typically made in strips to match the mortise, loose tenons are cut from the strips as needed; a simple hand saw can be used. Loose tenon joints are strong because of the large tenon section through the joint and generous side-grain gluing surface needed for long life.
Many devices have been made for producing mortises in wood. The devices fall into three general categories: hand tools, machine tools, and drilling jigs. Mortising chisels have been used by craftsmen for decades to hand cut mortises for joinery and hardware. A mortising chisel is a heavy bladed tool designed to be struck with a mallet. The process is slow and requires skill to perform accurately.
Machine tools such as the hollow chisel mortiser and chain mortiser greatly speed the cutting process. The hollow chisel mortiser plunges a drill bit surrounded by a square hollow chisel into the wood producing a square hole; indexing the workpiece with each stroke of the chisel produces the mortise. The chain mortiser, suited to production work, manipulates a miniature chain saw chain and bar into the wood producing the mortise with a single pass. These tools are costly and are not designed to mortise into the ends of long or bulky workpieces as loose tenon joinery often requires. Router based machine tools have become the common method for cutting mortises for loose tenon joints. They can be used by either manipulating the tool along a jig affixed to the workpiece or mounting the router to a machine with a movable table. The stock is clamped to the table and the workpiece guided into the router tooling. Both methods produce a milled mortise with rounded corners; tenon stock is then prepared with corresponding round corners. The process is dirty and very noisy. In addition, the milled mortise is limited in depth by the length of router tools available. Also, manipulating the router (or wood if a stationary router is used) is difficult on long or large pieces.
Various drill guide jigs have been available for boring wood. Although most jigs guide a single drill for the purpose of producing a doweled joint, some jigs are designed to allow boring of a series of adjacent, overlapping holes. These jigs are intended to hog material prior to hand chiseling. Installing deep mortise door locks is a typical application.
The objective of the present invention is to produce an improved loose tenon joint with deeper multiple arcuate mortise surfaces for increased side grain gluing surface. The present invention provides for a low cost jig enabling the woodworker to easily produce accurate, deep mortises for the joint (even on long or bulky workpieces) using only a pistol drill. Additionally, the present invention contemplates the provision of router and/or shaper tooling to produce the tenon material on these conventional machine tools.
As previously described, various forms of loose tenon joinery exist in prior art. None of these variations allow for added side grain glue surface by incorporating the multiple arcuate surfaces described by the present invention, nor do they mortise in a single operation using only a pistol drill. Prior art router based tools are limited in mortise depth by the length of the router bit. In addition, accurately milling the end of a long workpiece is difficult using router tools. This invention uses a conventional drill bit to accurately produce a loose tenon joint of greater depth, increasing side grain glue surface, while the material is stationary. Prior art hollow chisel and chain mortisers do not have capacity to mortise the ends of long lengths commonly needed in loose tenon joinery.
Other wood boring drill jigs exist in prior art. One of the earliest concepts was illustrated in U.S. Pat. No. 62,491 granted J. Isenberg on Feb. 26, 1867. The tool provided a series of laterally spaced guide holes for receiving and supporting drill bits. U.S. Pat. No. 310,710 (Nichols) also provided a bit for guiding drilled openings with particular application for boring doors for locks. Again, U.S. Pat. No. 1,432,485 (Moller) illustrated a jig usable in assisting the drilling of a mortise for insertion of a door lock. This jig merely provided a gauge to ensure alignment of several drilled openings to be later chiseled to produce a square-cornered mortise. U.S. Pat. No. 3,211,026 (Calahan) discloses a jig body with multiple crescent shaped guide holes used to guide a drill bit for fast material removal prior to hand chiseling. This design does not provide the full drill bit guide contact of the present invention for improved hole orientation. Goggins jig, UK Patent 1,314,809 maintains full drill bit contact but requires that the user correctly set spacing between each drilled hole; this is inaccurate and slow. The present invention provides full drill bit contact area and preset hole spacing for maximum accuracy. In addition, other hole sizes and spacing, can be easily substituted. Gumbrell UK Patent Application, discloses a jig with a movable guide bar. The guide bar has a drilled bit guide hole and index holes (pinned through the jig body) that control drilled hole spacing. Gumbrell does not disclose the simple construction of the present invention, nor does it allow for fast drilling with multiple guide holes.
Additionally, the Gumbrell device does not lead the user through the correct drilling sequence; the drill will "walk" if a hole is attempted next to a previously drilled hole. The present invention solves the problem encountered in the prior art, while simplifying the structure and performing more functions. The inventor is not aware of any prior art structure that teaches or discloses the invention disclosed and claimed herein.